Arrangement for antenna transmission lines



Nov. 7, 1933. G. w. GILMAN ARRANGEMENT Fon ANTENNA TRANSMISSION LINESOriginal Filed Sept. 6. 1930.

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Patented Nov. 7, 1933 PATENT orrlcs ARRANGEMENT FOR ANTENNA TRANS-MISSION LINES l George W. Gilman, Lawrenceville, N. J., assigner toAmerican Telephone and Telegraph Company, a corporation of New YorkApplication September 6, 1930, Serial No. 480,246 Renewed July 6, 193313 claims. (01.' 17e-44) This invention relates to arrangements forantenna transmission lines, especially such as are used at stationswhere a plurality of antenna adjusted for different frequencies arepresent, and

is applicable to both transmitting and receiving stations.

Its purpose is that of enabling a number of such antennae systemsdesigned for different frequencies to be connected so that the frequencyl0 of the entire station may be changed simply by changing the frequencyof the wave used. Another purpose is to accomplish this without the useof mechanical or electrical switching means calling for any movingparts. A further purpose is to devise an arrangement cf thiskind whichshall be equally applicable to transmitting and to receiving stations orto combinedv transmitting and receiving stations.V

The invention will be better .understood by reference to the followingspecica'tion and accompanying drawing, in which Fig. V1 shows onegeneralized method of accomplishing the results; Fig. 2 shows anothermethod; Figs. 3 and 4.s how specific ways in which the arrangements ofFigs. l and 2 may be carried out; and Figs. 5 and 6 are diagrammaticsketches to be used in the theoretical considerations of my invention.

Referring more particularly to Fig. 1, there is shown at 10 a source ofradio frequency power to be transmitted or a circuitfor receiving radiofrequency signals. In the immediate description to follow, however, theportion 10-will be treated as a transmitter. Running out from thistransmitter 10 is a transmissionline 11 which fans out to a number ofbranches 13, 14 and 15, eachof these branches going to an antenna systemof a particular frequency. Thus, for instance, the branch 13 suppliespower to an antenna system A. Similarly, the branches 14 and 15 supplypower to antennae systems B and C. These antennae systems are shownschematically only and are showneach as of a different form solely topoint out that this invention may be applicable to any type of antennasystem, suitable provision being made as by means of transformers toestablish the proper relationship between the impedance of thetransmission lines and the antenna systems.

Lacking any provision to the contrary, currents of a given frequencyemanating from the transmitter 10 would travel out on each of the branchtransmission lines and would tend to set up currents of greater or lessmagnitude in each and all of the antennae systems. This would give riseto certain losses of energy and certain irregularities, such asinterference from the Waves sent out from the different antenn. To avoidsuch effects it has been common practice to use separate transmissionlines going to the antenna system to be used with suitable selectiveswitching mechanism. In this invention, however, I propose to avoid suchsteps by arranging or modifying the characteristics of the transmissionline,

`and in particular the branches to the antenna systems, in such a mannerthat when the transmitter is tuned for the frequency corresponding toone particular antenna, no radio frequency power will be transmitted toanyantennze by the branch transmission lines except the one going to theantenna to be used.

One method ofaccomplishing this result is shown in 1 and consists inbridging across each branch line an impedance of a character which willprevent the. transfer to the antenna of power of alloperatingfrequencies except the one fcr'which its antenna is tuned. Thus, inbranch 13 there are shown bridged across the line two impedances Zt andZC which will prevent power transfer over the branch 13 of waves of thefrequencies corresponding to antennae B and C. Similarly, in branch 14there are bridged impedances Za and ZC which will prevent transfer ofthe frequencies corresponding to antenn A and C, and iinally, in thebranch 15 there are bridged the impedances Za. and .'Zb acting toprevent transfer of the frequencies appropriate to antenna A and B. rThephysical characteristics of these bridged impedances or networks mayVary considerably but in general will be of such a character that inorder to perform their function tothe bestadvantage they should each beplaced ata distance from the common junction point of the branches equalto 1/4 wave length or an odd number ofl 1/1. wave lengths of the wavewhich they are intended to suppress. This disposition of the impedanceshas been shown in Fig. 1 and the theoretical basis for this relationshipwill bev shown hereinafter.

Instead of using bridged or shunted networks it is possible to useseries networks in an analo- 100 gous manner and performing the samefunction. Such an arrangement is shown in Fig. 2 wherein branch 13 thereare included in series the impedances Zb and ZC acting to preventtransfer of power of the frequencies appropriate to 105 the antenna Band C. Similarly, impedances are connected in series in the branch lines14 and 15. In this case, for the most effective functioning of theseiinpedances it is desirable that they should be placed at a distancefrom the 110 common junction point of approximately 1/2 wave length or awhole number of 1/2 wave lengths of the wave which is not to betransmitted over any one branch.

As stated above, the particular form which the impedances or networksmay take on varies considerably, but the simplest form which may be usedfor a shunt arrangement is that shown in Fig. 3 in which each shuntconsists of a series inductance and capacity of such values as to betuned to the particular frequency of one of the other antenn. Similarly,in the series impedance the simplest network is that shown in Fig. 4,where each network consists of a socalled anti-resonant tuned circuit ora parallel tuned circuit. Such a circuit, as is well known, has theproperty of offering a very high impedance to a series E. M. F. offrequency equal to that for which the parallel circuit is tuned and arelatively small impedance for all other frequencies.

The theoretical considerations on which my circuit depends can bebriefly given by reference to Fig. 5 in which there is shown a smoothtransmission line Whose characteristic or iterative impedance is Z0, andwhich is terminated at some .point in that impedance. Across this lineat some point P there is shunted the impedance Zsh. Under thesecircumstances the input impedance as measured from the point 1 is Z1=Zatan hi where l is the position angle at the input of the line. Theposition angle 61 is related to the Y position angle at any point P bywhere 0 is the angle of the linebetween point P .and the sending end ofthe line. But

Y and 511:0, so that Z1=Zo tarlh 0 If now the distance from the inputterminals of the line to the shunt is an odd multiple of 1A; wavelengths for this particular frequency, then where Thus it is apparentthat at this particular frequency the impedance of this branch isinfinitely large andthe branch may therefore be bridged across othercircuits without loss and without absorbing energy from these circuits.A shunt which will offer zero impedance at a particular frequency isthat of a series tuned circuit comprising an inductance and capacity ofsuitable magnitudes, as shown in Fig. 3. At a frequency appreciablyremoved from its tuning frequency Zsh will be far from zero in value andZ1 will therefore be far from infinity. The effect of the shunt at suchother frequencies is therefore negligible. Additional shunts may beplaced across the line L, each tuned to the frequency for which it is toact as a suppressor, it being understood that each bridge should beplaced at a distance from the common junction point of an odd number of1/4 wave lengths of the wave to which the particular shunt applies.`

If in this case the series impedance Zser is made infinite at aparticular frequency, then CD p=tanh1a= and Z1=Zo 00th 0 If the distancefrom the input terminals of the 'line to the series impedance is anyWhole multiple of 1/2 wave length, then Z1=Zo coth ynfrziy'w and thus itis 'seen'that the same results are obtained as foi the shunt.

While the bridge cr series impedances have been shown as of a verysimple form, it is tov be understood that these may become more complexnetworks to yield similar results. Also it is to be understood that theresistance in these impedances should be preferably as low as possiblein order the more closely to conform to the conditions specified in thetreatment above. It should also be pointed out that while the positionsfor these impedances have been definitely speci-` fied. such positionsare not extremely critical and a reasonable variation from thebestvtheoretical position is permissible without serious effects.

Although in Fig. 1 the antenn arev shown as operating on the frequenciesfa, fb and fc, it is to be understood that they arenot necessarilytunedrto these frequencies, respectively. They ymay be so tuned or theymay be otherwise adjusted to operate favorably at these frequencies,and-then again, they may be aperiodic, the prime condition for thesystem being that the power which is intended for radiation on oneantenna shall not be able to get out to the other antennae. This is madepossible by the networks connected to the respective branch transmissionlines.

While in` Figs. 1 to 4 three branch lines have been shown, it is tobeunderstood that additional lines may be used according to the number ofadditional antennae, in which case it would be necessary to introduceadditional shunt or series impedances. In general each branch would havean impedance for each of the other branches. While the invention mayfind its chief usefulness at a transmitter station, where it enables achange in sending frequency to be made, with the appropriate antennaalone coming into action and Without any special switching mechanism,the invention will apply equally well to a receiving stationthatparticular branch transmission line alone coming into action which isappropriate for the Wave which is being received.

What is claimed is:

1. In radio transmission a plurality of antenn, a transmission linecomprising a pair of conductors, a branch therefrom to each antenna, andmeans in each branch to prevent transfer of power at the frequencies forwhich the antennae associated with the other branches are to beoperated, the branches being connected in parallel.

2.In a radio station comprising a plurality of antenna each to operateat its ownfrequency,

a transmission line comprising a pair of conn ductors and a branchtherefrom to each antenna, and networks in one branch kto prevent powertransfer thereover of those frequencies for which' the antenn of theother branches are tooperate.

3. In a radio station comprising a plurality of antennae each to operateat its own frequency, a transmissionline comprising a pair of conductorsand a branch therefrom to each antenna, and networks in each branch toprevent power transfer thereover of those frequencies for which theantennae of the other branches are to operate.

4. In a radio station comprising a plurality of antenn each adapted tooperate at its own frequency, a transmission line comprising a pair ofconductors and a branch therefrom to each antenna, and means forexcluding from each branch the frequencies of the antenn associated withthe other branches, said means comprising a plurality of networks eachtuned to one of the y frequencies to be excluded.

5. In a radio station comprising a plurality of antennae each adapted tooperate at its own frequency, a transmission line comprising a pair ofconductors and a branch therefrom to each antenna, and means associatedwith each branch for excluding from each branch the frequencies of theantennae associated with the other branches, said means comprising aplurality of networks each tuned to one of the frequencies to beexcluded.

6. In a radio station comprising a plurality of antenn each adapted tooperate at its own frequency, a transmission line and a branch therefromto each antenna, and means associated with each branch for excludingfrom each branch the frequencies of the antenn associated with the otherbranches, said means comprising a plurality of networks each tuned toone of the frequencies to be excluded, and each network being placed ata definite portion of a wave length from the junction point of thebranch with the main transmission line.

7. In a radio station comprising a plurality of antennae each adapted tooperate at its own frequency, a transmission line and a branchtransmission line comprising a pair of conductors therefrom to eachantenna, and a series tuned circuit bridged across one branch and tunedto the frequency at which another antenna is to operate. Y l

8. In a radio station'comprising a plurality of antennae each tuned toits own frequency, a transmission line and a branch therefrom to eachantenna, and a series tuned circuit bridged across one branch and tunedto the frequency of another Vantenna and placed at an odd number of 1A;Wave lengths at this frequency from the junction point of the branch tothe main transmission line.

9. In a radio station comprising a plurality of antenn each tuned to itsown frequency, a transmission line and a branch transmission linecomprising a pair of conductors therefrom to eachantenna, a plurality ofseries tuned circuits v bridged acrosseach branch and eachl tuned to thefrequency of the antenna of another branch. 10. In a radio stationcomprising a plurality of antennae each adapted to operate at adifferent assigned frequency, a transmission line and a branchtransmission line comprising a pair of conductors therefrom to eachantenna, a plurality of parallel tuned circuits connected in series witheach branch, each parallel tuned circuit in each branch being tuned tothe frequency corresponding to the antenna of another branch.

11. The combination of claim 5 characterized by the fact that theplurality of networks are so positioned with respect to the junctionpoint of the transmission line and the branches as to make any onebranch offer a high impedance, as seen from the junction point, tofrequencies to be excluded.

12. The combination of claim 5 characterized by the fact that eachnetwork is placed at a distance from the junction point of thetransmission line and the branches equal to a whole number of quarterwaves of the frequency it is designed to exclude.

13. The combination of claim 10 characterized by the fact that eachlparallel tuned circuit is placed at a distance from the junction pointof the transmission line and the branches equal to a whole number ofhalf waves of the frequency it is designed to exclude.

GEORGE W. GILMAN.

